Aromatic hydrocarbon mixtures are used on a large scale as gasoline. By distillation and extraction, individual aromatic compounds can be isolated from them such as benzene, toluene, the xylenes, and ethylbenzene which are used on a large scale as base materials for the chemical industry. Until recently, aromatic hydrocarbon mixtures suitable for the above-mentioned applications were prepared mainly by catalytic reforming of aliphatic hydrocarbon mixtures.
In an investigation concerning the preparation of aromatic hydrocarbon mixtures from aliphatic compounds, it was found that certain crystalline silicates which were recently synthesized for the first time, and described in Netherlands Patent Application No. 7,613,957 incorporated herein by reference, are suitable for use as catalyst for the preparation of aromatic hydrocarbon mixtures from aliphatic oxygen-containing hydrocarbons having the general formula C.sub.n H.sub.m O.sub.p. By contacting a compound of this type at an elevated temperature with a catalyst containing one of these crystalline silicates, a hydrocarbon mixture is obtained which can be separated into a mixture of aliphatic hydrocarbons with from one to four carbon atoms in the molecule and an aromatic hydrocarbon mixture of which the components contain five and more carbon atoms in the molecule.
The suitability of a catalyst for the preparation of an aromatic hydrocarbon mixture from aliphatic oxygen-containing hydrocarbons having the general formula C.sub.n H.sub.m O.sub.p is judged by the activity and the selectivity which are defined as follows. Activity is a measure of the capacity of the catalyst to convert the compound C.sub.n H.sub.m O.sub.p into a mixture of hydrocarbons. The activity of the catalyst is higher according as more hydrocarbons are formed. Selectivity is a measure of the capacity of the catalyst to convert the compound C.sub.n H.sub.m O.sub.p into hydrocarbons with five and more carbon atoms in the molecule. The selectivity of the catalyst is expressed as the weight percentage of produced hydrocarbons with five and more carbon atoms in the molecule calculated on the total amount of hydrocarbons produced. A catalyst is judged to be more suitable for the preparation of an aromatic hydrocarbon mixture from aliphatic oxygen-containing hydrocarbons having the general formula C.sub.n H.sub.m O.sub.p according as activity and selectivity are higher.
Continued investigation concerning this subject by the applicant has shown that the use of the crystalline silicates for the present purpose using certain oxygen-containing hydrocarbons as the starting material yields much better results than when use is made of other oxygen-containing hydrocarbons, in spite of the fact that they will satisfy the above-mentioned general formula. It has been found that oxygen-containing hydrocarbons having the general formula C.sub.n H.sub.m O.sub.p can, according to their behavior in the conversion into an aromatic hydrocarbon mixture in the presence of crystalline silicates as the catalyst, be divided into two groups, namely compounds for which m-2p/n is greater than 1 and compounds for which m-2p/n is at most 1. When the crystalline silicates are used as catalysts in the conversion of compounds for which m-2p/n is greater than 1, these catalysts show a high activity and selectivity. When the crystalline silicates are used as catalysts in the conversion of compounds for which m-2p/n is at most 1, these catalysts show so low an activity that they are unsuitable for use in the conversion of these compounds on a technical scale.
Surprisingly, it has now been found that the crystalline silicates can nevertheless be used successfully as catalysts in the preparation of an aromatic mixture from the compounds for which m-2p/n is at most 1 if these compounds are mixed with a predominant molar amount of one or more of the compounds for which m-2p/n is at most 1. It has been found that when the crystalline silicates are used as catalysts for the preparation of an aromatic hydrocarbon mixture from the above mixtures of the two types of oxygen-containing hydrocarbons, the catalyst shows a high activity and selectivity.